Multi-hop relaying is drawing much attention as a technology that can expand cell coverage at a low cost in a cellular system and provide users with high transmission rate. Representatively, IEEE 802.16j relay task group is standardizing mobile multihop relay (MMR), and developed laboratories and enterprises studying 4th generation (4G) mobile communications around the world are competitively developing the multi-hop relaying technology.
For the multi-hope relaying, one or more relay stations that relay data between a base station (BS) and a mobile station (MS) are needed. In the cellular system, a relay station (RS) may be a separate device separated from a BS and an MS, or the MS may serve as an RS for another MS. The RS reduces path loss by relaying data between a BS and an MS, thereby enabling high-speed data communication. Also, because the RS transmits a signal even to a distant MS or an MS in a shadow area, the cell coverage can be expanded. In other words, as the RS is disposed in the system as illustrated in FIG. 1A, a transmission rate can be improved at a cell shadow area and a cell boundary. Also, as the RS is disposed in the system as illustrated in FIG. 1B, the coverage can be expanded.
FIGS. 2A and 2B illustrate the use of radio resources in both cases of direct transmission and transmission using N-1 relay stations, and in FIGS. 2A and 2B, hops are divided on a time-division basis. FIG. 2B illustrates an example of the use of radio resources in a general relay broadband wireless communication system. When a signal is transmitted from a base station (BS) to a mobile station (MS) by using the relay station (RS), consumption of radio resources occurs as illustrated in FIG. 2B. As shown in FIGS. 2A and 2B, when data of one unit is transmitted over N hops, the transmission time that is N times longer than the direct-transmission time is required, that is, system resources are consumed N times more than in the direct transmission.
In the cell configuration illustrated in FIGS. 1A and 1B, each RS receives data from a specific BS, and the received data is transmitted to mobile stations within RS coverage. The RS cannot perform simultaneous transmission and reception using the same frequency. Thus, different frequencies or time resources must be allocated to a link between the BS and the RS and a link between the RS and the MS. The radio resources being consumed in communication between the BS and the RS act as a huge overhead, thereby reducing the amount of usable resources in communication between the BS and other mobile stations.
A plurality of relay stations fail to provide smooth services if sufficient resources are not ensured in the link between the BS and the RS and the link between the relay stations. Also, an increase in system capacity cannot be expected because of the overhead caused by resource consumption of the plurality of relay stations. Furthermore, to allow the relay service in a wide area, a large number of relay stations per BS are needed in the cell configuration of FIGS. 1A and 1B. Thus, the RS installation cost increases.